1. Field of the Invention
The present invention relates to a polishing apparatus for a notch portion formed on the periphery of a wafer (hereinafter, which is simply called "a polishing apparatus") which is used for positioning adjustment or crystal orientation adjustment of the wafer.
2. Description of Related Art
Generally, a photolithographic technique is used for the purpose of forming a pattern for a semiconductor integrated circuit in a single crystal of Si, compound semiconductor or the like (hereinafter, which is simply called "a wafer"). Such an application of the photolithographic technique requires a precise positioning adjustment and a precise crystal orientation adjustment. For these requirement, a linear portion is generally made on one side of the periphery of the wafer in order to use the linear portion as a standard for the positioning adjustment and the crystal orientation adjustment. This linear portion of the wafer is called an orientation flat.
When such an orientation flat is formed on the wafer, a portion of the periphery of the wafer is cut off linearly. Therefore, the area of the cut portion of the wafer is large, so the number of semiconductor chips which can be obtained from a wafer decrease. As a result, according to such a conventional method in which an orientation flat is formed on a wafer, it is impossible to effectively utilize an expensive wafer. The conventional method has another problem in that it is difficult to treat wafers when each of the wafers has a large diameter and an orientation flat, so the wafers are not properly in balance, for example, on a work using a spin dryer or the like in which the wafers are dried by a centrifugal force due to a high speed rotation.
Recently, in order to solve the above problems, another method has been used in which a small notch having a circular arc shape or a shape of a character "V" in plan view is formed in a portion of the periphery of each wafer. In this method, positioning and crystal orientation adjustments of the wafer are carried out by using the notch.
Such a notch is formed by using a grinding wheel 31 or the like, as shown in FIG. 3. That is, the notch portion 32 is formed by making the grinding wheel 31 press into a portion of the periphery of the wafer W and by grinding the portion, as shown in the plan view of FIG. 4. FIG. 5 is a vertically cross-sectional view showing the wafer W cut along the line 5--5 of FIG. 4. The wall of the wafer W in the notch portion 32 swells into a curve toward the outside in the middle of vertical direction, as shown in this Figure.
In a photolithographic process, particles may be a main impact to form fine patterning for semiconductor devices. Therefore, in order to minimize the amount of the particles, a clean room having a higher cleanliness is required. Furthermore, it is desired to suppress generation of particles from the wafer as less as possible.
In order to solve the above problems, it is necessary to polish the wall of the wafer in the notch portion 32 and thereby to prevent generation of particles when the wall of the wafer in the notch portion 32 is in contact with a hard pin for crystal orientation adjustment of the wafer. However, the width of the notch portion 32 to be formed is smaller than that of the orientation flat. As mentioned above, the notch portion 32 has a circular arc shape or a shape of a character "V" in plan view and the wall of the wafer W in the notch portion 32 has a complicated shape. Therefore, it is difficult to polish this portion.